1. Field of the Invention
The present invention relates to a cathode catalyst layer and a membrane electrode assembly (MEA) for a fuel cell. More specifically, the present invention relates to a manufacturing method of a cathode catalyst layer and MEA which provide a resultant fuel cell with a high level of power generation performance with use of a non-noble metal (or, non-platinum) catalyst having oxygen reduction activity.
2. Description of the Related Art
A fuel cell is a power generation system which produces electric power along with heat. A fuel gas including hydrogen and an oxidant gas including oxygen react together at electrodes containing a catalyst in a fuel cell so that a reverse reaction of water electrolysis takes place. A fuel cell is attracting attention as a clean energy source of the future because of advantages such as high efficiency, a small impact on the environment and a low level of noise relative to conventional power generation systems. A fuel cell is classified into several types according to an ion conductor employed therein. A fuel cell which uses a proton conductive polymer membrane is called a proton exchange membrane fuel cell (PEMFC) or a polymer electrolyte fuel cell (PEFC).
Among various fuel cells, a PEMFC (or PEFC), which can be used at around room temperature, is regarded as a promising fuel cell for use in vehicles and household stationary power supply etc. and is being developed widely in recent years. The PEMFC (or PEFC) includes a membrane electrode assembly (MEA) in which a pair of electrodes is arranged on both sides of a polymer electrolyte membrane. In the PEMFC (or PEFC), the MEA is interposed between a pair of separators, on each of which either a gas flow path for supplying a fuel gas containing hydrogen to one of the electrodes or a gas flow path for supplying an oxidant gas containing oxygen to the other electrode is formed. While one of the electrodes for supplying the fuel gas is called an anode (or fuel electrode), the other electrode for supplying the oxidant gas is called a cathode (or air electrode). In general, each of these electrodes includes a catalyst layer in which a polymer electrolyte and catalyst loaded carbon particles are contained and a gas diffusion layer which has gas permeability and electrical conductivity. A noble metal such as platinum etc. is used as the catalyst in the catalyst loaded carbon particles.
Apart from other problems such as improving durability and output density etc., cost reduction is the most major problem for putting the PEMFC (or PEFC) into practical use.
Since the PEMFC (or PEFC) at present employs expensive platinum as the electrode catalyst, an alternate catalyst material is strongly desired to fully promote the PEMFC (or PEFC). As more platinum is used in the cathode (air electrode) than in the anode (fuel electrode), an alternative to platinum (namely, a non-platinum catalyst) with a high level of catalytic performance for oxygen-reduction on the cathode is particularly well under development.
A mixture of a noble metal and nitride of iron (a transition metal) described in Patent document 1 is an example of a non-platinum catalyst for the cathode. In addition, a nitride of molybdenum (a transition metal) described in Patent document 2 is another example. These catalyst materials, however, have an insufficient catalytic performance for oxygen-reduction in an acidic electrolyte and are dissolved in some cases.
Non-patent document 1 reports that a partially-oxidized tantalum carbonitride has both excellent stability and catalytic performance. This oxide type non-platinum catalyst has a high level of catalytic performance for oxygen-reduction in itself as a catalyst.
Regarding conventional platinum loaded carbon catalyst, Patent document 3 teaches a technique of improving catalyst-use efficiency by providing the carbon surface with a hydrophilic group so as to improve affinity for the proton conductive polymer electrolyte. In addition, Patent document 4 teaches a surface modified carbon which protects the surface from oxidation when a hydrophilic group is introduced to the surface of the carbon of a conventional platinum loaded carbon catalyst.    <Patent document 1>: JP-A-2005-44659.    <Patent document 2>: JP-A-2005-63677.    <Patent document 3>: JP-A-2006-4662.    <Patent document 4>: JP-A-2007-161511.    <Non-patent document 1>: “Journal of The Electrochemical Society”, Vol. 155, No. 4, pp. B400-B406 (2008).